1. Field of the Invention
The present invention relates to image enhancement in X-ray angiography and more particularly to generating enhancement masks at different scales and then combining the enhancement masks together using a pyramid scheme hence enabling contrast improvement for image structures of different sizes.
2. Description of the Prior Art
Since the introduction of conventional angiography and digital subtraction angiography techniques, the examination and diagnosis of the arterial vascular system has become easier and more apparent for physicians. These techniques are now widely used in interventional radiological diagnosis and surgery. Despite advances in imaging technology, blood vessels in X-ray angiography often exhibit low intensity contrast with respect to their surrounding soft tissues. The problem is particularly severe for fine vessel structures.
A major challenge for enhancement is the ability to emphasize vessel structures without creating artifacts. Edge overshooting and noise magnification are among the main sources of artifacts resulting from existing enhancement methods used in clinical settings, such as the unsharp-masking method. In addition, the effectiveness of this and other methods is often limited to vessel structures at a predetermined scale.
U.S. Pat. No. 5,467,404 assigned to Agfa-Gevaert and entitled Method And Apparatus For Contrast Enhancement discloses enhancing a digital image by decomposing said original image into a sequence of detail images or into an array of coefficients representing detail strength at multiple resolution levels and a residual image, modifying each pixel of each detail image or each detail coefficient according to at least one non-linear monotonically increasing odd conversion function and constructing a processed image by accumulating detail obtained from the modified detail images or from the detail coefficients and further adding the residual image. This approach is aimed at enhancing, locally or globally, the dynamic range within the image (i.e. selected subband). Histogram equalization (globally or locally adaptive) is the method cited for the non-linear conversion of pixel values to achieve the enhancement objective.
It is an object of the present invention to provide a powerful computer aided perception system for physicians which can provide enhanced images for optimal visualization of blood vessels and consequently facilitate the extraction of valuable diagnostic information from angiographic images.